Many surgical instruments utilize a high pressure fluid jet to rotate an end effector, such as a cutting tip for cutting tissue or bone. The cutting tip is coupled to a shaft that is mated to a rotor. Fluid passing from an inlet to an outlet of a cylinder containing the rotor causes the rotor rotate, thereby rotating the shaft and the cutting tip. In use, when the tip is placed in contact with tissue, a load or counter-torque applied to the tip will reduce the rotational speed of the shaft. Thus, the fluid must be driven through the system at relatively high pressures to maintain the cutting tip at a speed sufficient to effectively cut tissue. When the cutting tip is separated from the tissue, however, the speed of the cutting tip will increase. This can compromise visualization of the surgical site, as high speed rotation of the cutting tip can cause debris and bubbles to mix and form in the surrounding environment. The air bubbles created in the fluid environment, along with floating debris, can also interfere with any optical equipment positioned at the surgical site.
While surgeons could reduce the pressure of the fluid jet, or deactivate the fluid jet, to thereby reduce or eliminate the rotational speed of the cutting tip when the cutting tip is not in contact with tissue, this can be cumbersome on the surgeon, especially when the cutting tip is repeatedly repositioned relative to the tissue. Accordingly, there remains a need for improved fluid jet-driven instruments, and in particular for methods and devices for controlling the rotational speed of shaft on a fluid jet-driven instrument.